1. Field of the Invention
The present invention relates to a sputtering apparatus and a sputtering method for obtaining a uniform film and desired film thickness characteristics in a film formation on a substrate with surface irregularities, mounted on a substrate holder, and more particularly to a film forming apparatus and a film forming method for forming a uniform film with a desired film thickness distribution such as a multi-layered antireflection film or a multi-layered film mirror or an X-ray reflecting multi-layered film, on a substrate such as a large-aperture lens.
2. Description of the Related Art
A magnetron sputtering apparatus of parallel flat plate type, employed already widely, is an apparatus in which a target constituting a material of a thin film and a substrate mounted on a substrate holder are positioned in a mutually opposed manner in a vacuum chamber, and a plasma is generated to sputter the target whereby particles sputtered out are deposited on the substrate to form a thin film thereon, and has features simpler than other methods and capable of a high-speed film formation, a large-area film formation and a longer target life.
In relation to such sputtering apparatus, a sputtering in the field of optical films has been recently investigated, and, particularly in a semiconductor exposure apparatus such as a stepper, together with an improvement toward a higher NA for improving the exposure performance, there are requested an increase in the lens aperture, an improvement in inclined incident characteristics for a ray entering the lens, and, an inclined film of a large diameter and a high precision (for improving the inclined incident characteristics) in an X-ray (EUV) exposure apparatus of next generation.
In a multi-layered X-ray mirror of molybdenum (Mo) and silicon (Si) for an X-ray wavelength of 13.4 nm, the reflection characteristics are lowered depending on an incident angle of the incident X-ray entering the mirror surface, because of an extremely narrow bandwidth of the reflection characteristics. As a countermeasure, a method of forming reflecting mirror characteristics, matching the incident X-ray angle within the plane of the X-ray mirror, is being adopted. It is therefore necessary to highly precisely control the film thickness distribution of molybdenum (Mo) and silicon (Si) within the mirror plane.
For controlling the film thickness distribution, Japanese Patent Application Laid-open No. H10-30170 discloses a method of employing a shield mask and controlling a shape of such shield mask and a moving speed distribution thereof, thereby controlling the film thickness distribution.
Also Japanese Patent Application Laid-open No. H9-213634 discloses a method of film formation under a movement of the substrate in an X or XY direction parallel to an evaporation source.
Also U.S. Pat. No. 6,010,600 proposes an inclined film formation by a revolution speed control when a substrate, executing a revolving motion and a rotating motion, passes over a target.
However, optical elements employed in an exposure apparatus, etc. are designed for increasingly shorter wavelengths and in various shapes such as an aspherical surface, an arbitrarily curved surface and a parabolic surface. For this reason, a film thickness control of a higher precision than in the prior technology is being requested.
For realizing an optical element requiring such highly precise film thickness control, the following film forming apparatus and film forming method are indispensable:                (1) Ability for film thickness distribution control for various irregular shapes;        (2) Adaptability to plural targets (showing different film thickness distributions by a difference in an emission angle distribution of a material);        (3) A film formed by an inclined entry of sputtering particles has a low film density, thus showing a change in the refractive index and a change in the optical characteristics by a moisture adsorption. Therefore the film forming apparatus should minimize an inclined entry component of the sputtering particles;        (4) A load lock type film formation should be possible in order to minimize particle generation, and influence of residual gas, particularly water;        (5) The film forming apparatus should not introduce a complex moving mechanism into the vacuum chamber.        
In the aforementioned method of employing a shield mask and controlling the shape of the shield mask and a moving speed distribution thereof, thereby controlling the film thickness distribution (Japanese Patent Application Laid-Open No. H10-30170), it is difficult to meet the above requirements (1) and (2) with a mask of a single shape, there is required an operation of replacing the mask by opening the vacuum chamber to the air. The opening of the vacuum chamber to the air causes water in the air to be adsorbed in a film deposited in the vacuum chamber, thereby increasing a stress in the film and inducing a peeling thereof, thus resulting in an influence corresponding to (4). Also the shield mask, positioned in the vicinity of the substrate between the target and the substrate, undesirably becomes a source of particles.
The method of employing a shield mask and controlling the shape of the shield mask and a moving speed distribution thereof, thereby controlling the film thickness distribution, as disclosed in Japanese Patent Application Laid-open No. H10-30170, can only deposit a rotationally symmetrical film but cannot be adapted to a film of other configurations.
The inclined film formation by a revolution speed control when a revolving and rotating substrate passes over a target, as proposed in U.S. Pat. No. 6,010,600, can only deposit a rotationally symmetrical film.
The method of film formation under a movement of the substrate in an X or XY direction parallel to an evaporation source, disclosed in Japanese Patent Application Laid-open No. H9-213634, in a film formation of a peripheral portion of a substrate having a large curvature in an irregularity and/or a complex shape, brings about a large inclined entry component of the sputtering particles entering the substrate, thereby resulting in a film of a low film density and causing a shift in the characteristics. Further, a complex moving mechanism is undesirably required in the vacuum apparatus.